Determining a location and identity of a buried asset without accurate written records can be a challenging task. In some cases, determination of the location is performed by systematically digging holes in ground until the asset is found. In other cases, ground penetrating radar (GPR) is used, assets being located by detecting a signal reflected by the asset. (Reference to GPR includes radiation having a frequency in the range of from around 200 MHz to around 1 GHz).
Use of GPR for locating buried assets suffers the disadvantage that radiation can be reflected by a number of features of a volume of ground, including variations in moisture content, solids composition, the presence of wildlife, and voids formed for example by tunnelling wildlife. Thus it can be difficult to reliably identify a location of a buried asset.
Acoustic techniques have also been used to identify buried assets and found to suffer similar disadvantages to GPR techniques.
It is also known to attach a radio frequency identification tag (RFID tag) to a non-buried asset such as items for sale in a store in order to enable identification of the asset using a suitable RFID tag reader. Typically, an RFID tag is placed in an inductive RF magnetic field to induce a flow of electrical current in the tag. The flow of current is used to generate an RF transmission by the tag to the reader.
Inductive magnetic fields are mostly in the near field, however, and the use of such RFID tag systems to locate and identify assets more than a few meters from the reader is generally not easy.
In the case of buried assets the problem is exacerbated since inductive RF electromagnetic waves are more strongly attenuated by soils than by air.
More recently, far-field RFID technology has been developed. However, RFID readers are incapable of generating sufficiently large excitation signals to detect RFID tags at depths below a surface at which many assets of commercial interest are buried.
U.S. Pat. No. 3,769,623 discloses a dichroic plate for passing radiation within a particular frequency band and reflecting radiation outside of that frequency band. The dichroic plate has slots provided therein dimensioned to pass radiation of a selected frequency.
U.S. Pat. No. 5,837,926 discloses mines having tuned passive electromagnetic resonants to enhance radar detection.
The UK HSE (Health Safety and Executive) Enforcement Policy for Replacement of Iron Gas mains 2006 has developed legally binding requirements on the UK gas distribution network operators for the replacement of ageing cast iron gas mains pipes with pipes formed from plastics materials.
However, current methods used to trace plastics-based pipes are either inefficient or very expensive. This hampers replacement and repair operations leading to unnecessary delays. Excavations often result in accidental damage to third party assets, casualties and traffic congestion. Furthermore, the Traffic Management Act and Records Code of Practice for buried assets largely suffer from inaccurate records.
It is desirable to enhance the visibility of buried assets such as plastics pipes.